The digital mobile telephone system which forms the basis for the so-called D-Network used in Germany is generically designated by the acronym GSM, which comes from the French phrase for mobile special group (GROUPE SPECIALE MOBILE). In this system the radio transmission is purely digital, hence the name D-Network. It is a cellular network in the sense that there are many fixed stations with which a mobile station can communicate sequentially as it moves from the cell of one station to the cell of the next.
The receiving frequency region reserved for the D-Network stretches from 935 to 960 MHz and is sub-divided into 124 receiving channels each having a 200 kHz bandwidth. Each such frequency band is organized to provide eight participant channels in a time multiplex raster in which the individual time slots are similarly organized frame by frame.
As described in GSM 05.02, released January 1990, there are traffic channels (TCH's) and control channels. The former were sub-divided into encoded speech traffic channels and data traffic channels.
The control channels include frequency correction channels for frequency correction of a mobile station, synchronization channels for a frame synchronization of the mobile station and identification of a base transceiver station and broadcast control channels for general information from a base station. The latter may be organized as common control channels with certain blocks in each common control channel reserved for access grant messages.
A particular set of radio frequency channels is allocated to a particular cell, such a set being defined as the cell allocation (CA). One radio frequency channel of the cell allocation is used to carry synchronization information, known as the BCCH carrier, and serves as the broadcast control channel. A subset of the cell allocation allocated to a particular mobile station is designated as the mobile allocation (MA). A radio frequency channel is partitioned into time slots, so that transmission takes place in time division multiple access (TDMA) frames. There are eight time slots to a TDMA frame, which has a duration of a little less than 5 ms. At a base station the start of a TDMA frame on the uplink is delayed by the fixed period of three time slots from the start of the TDMA frame on the down link, while at a mobile station this delay will be variable to allow adjustment for a signal propagation delay. This adjustment is referred to as adaptive frame alignment.
With the above general information, a known synchronization system incorporated in GSM recommendation 05.10 can now be described.
A base station sends signals on a broadcast control channel to enable a mobile station to synchronize itself to the base station and if necessary correct its frequency standard so as to put it in line with that of the base station. The signal sent by the base station for these purposes are (a) frequency correction bursts and (b) synchronization bursts. A burst is defined as an interval within which the radio frequency carrier is modulated by a predetermined data stream. For such data a time slot is divided into 156.26 bit periods and the bits are numbered so that the timing of a burst within a time slot can be defined in terms of bit number. GSM 05.02 defines four full bursts of a useful duration of 147 bits and one short burst of a useful duration of 87 bits. The document just mentioned shows that a frequency correction burst (FB) begins with three tail bits, follows with 142 fixed bits and three more tail bits, with the remainder of the time providing a guard interval. The tail bits are zeroes and the "fixed bits" also, so that this burst is equivalent to an unmodulated carrier with a frequency offset above the nominal carrier frequency.
A synchronization burst begins with three tail bits, followed by 39 encrypted bits and then 64 extended training sequence bits, followed by encrypted bits, tail bits and a guard interval. The extended training sequence bits are a particular pattern of ones and zeroes.
The timing of time slots, TDMA frames, traffic channel frames and control channel frames are all related to a common set of counters which run continuously whether the mobile station and base station are transmitting or not. Thus once the mobile station has determined the correct setting of these counters all its processors are synchronized to the currently serving base station. The mobile station times its transmissions to the base station in line with those received from the base station. The base station sends to each mobile station a timing advance parameter according to the perceived round trip propagation delay. The mobile station advances its timing by this amount, with the result that signals from different mobile stations arriving at the base station are compensated for propagation delay.
The timing state of the signals transmitted is defined by the following counters:
quarter bit number QN (0-624) PA1 bit number BN (0-156) PA1 time slot number TN (0-7) PA1 TDMA frame number FN (0-22715647) PA1 QN is set by the timing of the training sequence, PA1 TN=0 when the sync burst is received, has a particular value (set forth in GSM rec. 05.10 page 3) when the sync burst is received. PA1 (1.1) coarse frequency synchronization, PA1 (1.2) coarse frame synchronization, PA1 (1.3) fine frequency synchronization, and PA1 (1.4) fine frame synchronization. PA1 (2.1) frame synchronization with fine frequency synchronization and PA1 (2.2) a preliminary processing of a data signal. The extended synchronization comprises of PA1 (3.1) a coarse frame synchronization and PA1 (3.2) a fine frame synchronization with fine frequency synchronization.
QN increments every 12/13 microsecond, BN is the integer part of QN/4, TN increments whenever QN changes from count 624 to 0 and FN increments whenever TN changes from count 7 to 0.
The mobile station can use a timing of the receipt of the synchronization burst to set up its time base counters as follows:
GSM recommendation 05.10 pages 4 and 5 sets very strict requirements regarding the carrier frequency transmitted by the mobile station and the accuracy to which the mobile station shall keep its internal time base in line with that of signals received from the base station.